The present invention relates to clamps for attaching a plate to a rotatable drum and, more particularly, to clamps for attaching plates of various sizes to the drum of a so-called external-drum platesetter that can operate at very high rotation speeds.
Imagesetters, namely marking engines for plotting images on plates or sheets of film, of the so-called external-drum type, are known in the art, Typically, a sheet is attached to the outer surface of a rotating drum and held in contact with the surface by a vacuum system, which has orifices distributed over the surface. Often, such an imagesetter is required to handle sheets of various sizes. U.S. Pat. No. 5,383,001 to Bosy, addresses such a need, by providing a suitable design of the vacuum system.
External-drum platesetters are a particular category of external-drum imagesetters, wherin the image is plotted on a light-sensitive printing plate. Printing plates are generally made of metal and therefore they are stiffer than films and their mass per unit area is much higher. Thus, when a plate is attached to the drum, it requires a greater force to keep it in contact with the drum's surface at any given rotational speed, to counteract the centrifugal force, than is required by a film. Consequently, a vacuum system to hold plates on the rotating drum must be more efficient and more powerful than that used for holding films. However, even with a powerful vacuum system, there is a danger of failure, due to a bend or a bulge at the edge of the plate. Moreover, since in a vacuum system the forces holding the plate are exerted by atmospheric pressure, they have an upper limit. Above a certain rotational speed the centrifugal force per unit area exceeds atmospheric pressure and any plate attachment system solely based on vacuum would then totally fail. Such failures may cause the entire plate to fly off the drum by centrifugal action--which would be disastrous for the entire machine. Therefore platesetters require mechanical attachment means--either to assist the vacuum system and guard against drastic failure, or as the main holding means, possibly still aided by vacuum. One common and useful such means is clamps mounted on the drum and operative to hold down the edges of the plate.
Clamps and grippers for holding a plate onto a rotatable drum have been known in the art. For example, clamps (also called grippers) are generally used in offset printing presses to hold the printing plate onto a cylinder. During operation, centrifugal forces act to cause a pulling force on each edge of the plate, such that tends to pull the edge from the clamp. Therefore in printing presses, clamps are designed to exert enough gripping force to exceed that required for countering the centrifugally induced pulling force, at maximum expected rotational speed, by a safety margin. Such clamps are disadvantageously cumbersome for mounting on platesetters and, moreover, difficult to activate (i.e. to open for accommodating the plate), because they must include either a heavy bolting or leveraging mechanism or a very strong spring-loading mechanism. A clamp on a printing cylinder must also exert a positive stretching force on the plate, in order to assure contact between the plate and the cylinder over its entire surface.
U.S. Pat. No. 3,203,074 describes an improved clamp, in which the gripping and stretching forces increase by centrifugal action in proportion to the rotational speed. A clamp with such a centrifugal action (also termed dynamic clamp) may overcome the disadvantages discussed in the previous paragraph. U.S. Pat. No. 4,250,810 discloses a further improved clamp with centrifugal action whereby the gripping and stretching forces can be independently adjusted.
It is noted that in platesetters, there usually is also a vacuum system, which assures contact between the plate and the drum over its entire surface, and that therefore no stretching force, but only a gripping, force, is required at the edges of an attached plate. U.S. Pat. No. 5,335,046 discloses a clamp for the drum of a platesetter with a mechanism that centrifugally induces a gripping force. There is a row of such clamps for the leading edge and another one for the trailing edge. The mechanism includes a weight, connected to the clamp by a rod, located inside the drum near the opposite surface. The clamp is opened by means of a piston and cylinder assembly, activated by supplied pressurized air. The clamp mechanism of the '046 patent has two serious drawbacks: (1) It is relatively complicated, with a relatively large number of parts, and thus is expensive to manufacture. (2) It does not conveniently accommodate plates of various sizes. The '046 patent does disclose the possibility of providing a row of trailing edge clamps for each plate size; however, such an arrangement is even more expensive and, furthermore, does not accommodate small variations in plate size. The secondly listed drawback also applies to other types of clamps of prior art, such as those discussed hereabove.
There is thus a widely recognized need for, and it would be highly advantageous to have, a clamping system, for attaching a plate to a rotatable drum, that is dynamic, i.e. has a centrifugally induced gripping action, is adaptable to any plate size and easy to operate and is of simple design, having a small number of parts.